1. Field of Invention
This invention relates to a method and device for controlled combustion of accidentally formed explosive mixtures contained within vacuum furnaces, in particular vacuum induction-melt furnaces and vacuum arc-melt furnaces, having at least one igniter and at least one ignition source.
2. Description of Prior Art
It has been a long-standing problem that explosions occur in the processing and casting of reactive metals, such as titanium and zirconium. For example, since the commercial development of the titanium processing industry in the early 1950's, there has been a continual problem of explosions with catastrophic loss of human life and/or capital equipment.
The explosions form under accident conditions. The accident begins when water comes in contact with the molten reactive metal. In the case of the vacuum arc-melting furnaces, one mode of failure occurs when the tip of a water-cooled non-consumable electrode blows off and high-pressure water is injected directly into the crucible containing the molten metal. Another failure mode occurs when the electrode arcs to the water-cooled crucible, melts part of the crucible, and releases the cooling water, which is under high pressure. A similar failure mode occurs in vacuum induction-melt furnaces when the induction field is not contained within the melt, locally melts part of the water-cooled crucible, and releases the high-pressure water. A copious amount of steam is produced when the water comes into contact with the molten metal, which can generate a pressure excursion by itself. Additionally, the molten metal chemically reacts with the steam, stripping the oxygen from the steam to form a metal oxide, and liberating large quantities of hydrogen. The furnace then contains only hydrogen and steam since the furnace was initially under a strong vacuum and originally contained no air. The hydrogen and steam, which are then above atmospheric pressure, are vented to the atmosphere through a venting device so that the hydrogen and steam are eventually relieved to atmospheric pressure. As the steam condenses and produces a slight vacuum, air is drawn into the furnace to create a potentially explosive hydrogen-air-steam mixture. An explosion occurs when the mixture comes into contact with a random ignition source, such as a spark or hot surface.
A number of “solutions” for each of these problems have been proposed and implemented but all have ultimately failed to prevent continued explosions from occurring in the furnaces and damaging equipment. For example, melting is now done with as short an arc as possible, controls have been installed to maintain clearance between the electrode and the crucible, and interlocks have been installed to shut down power if the water pressure is lost. However, explosions continue to occur because new failure modes are continually identified.
It eventually became apparent to the industry that, despite their best use of controls, explosions were always possible. It was apparent that the prevention of hydrogen explosions was an insoluble problem and the industry has resigned itself to protecting personnel and equipment, rather than eliminating the explosion per se. The furnaces are now constructed to withstand moderate internal pressure with appropriate venting to relieve internal pressure. A bunker with one frangible wall shields the furnace. The frangible wall, preferably an outside wall, is designed to blow out in the event of an explosion and direct the blast away from personnel and other equipment within the building. By use of optics, such as video cameras, the operators can be isolated from the furnace in control rooms. This “solution”, however, does not prevent the explosions and still results in serious damage to the furnace and loss of production from downtime.
The reactive metals processing industry has made many attempts to prevent hydrogen explosions over the past nearly 50 years, each attempt being unsuccessful in its ultimate objective because of the random cause of the accidents. There is a long-felt need to solve this seemingly insoluble problem but the industry has resigned itself to the existence of hydrogen explosions. The industry's current approach is to use the best controls available to reduce the probability of an explosion but recognize its possibility and confine the furnace to limit the effects of its damage.
A different industry, the nuclear power industry, must also address the possibility of a hydrogen explosion under severe accident conditions. Under degraded reactor core conditions, hydrogen can be injected under high pressure into a steamy air atmosphere in the nuclear reactor containment. The hydrogen injection location will be unknown and will depend on each accident scenario. Different means to control the hydrogen have been employed: (1) dilute the hydrogen with air contained in very large containments to render the mixture nonexplosive, (2) eliminate the hydrogen through catalytic recombination, and (3) consume the hydrogen through controlled combustion using igniters.
The unique accident conditions in the vacuum furnaces preclude the use of most techniques used in nuclear power plants. For example, in a nuclear power plant the initial composition of the mixture in the containment is just air and steam prior to the injection of hydrogen. The mixture is initially nonflammable (no fuel is present) and can remain nonflammable as the hydrogen is introduced into the containment given a large enough initial quantity of air to thoroughly dilute the hydrogen. In a vacuum furnace, on the other hand, the initial composition of the mixture is just hydrogen and steam. As the air is introduced into the furnace, the mixture would change from initially nonflammable (no oxidizer present), to explosive as sufficient air is added, and then back to nonflammable (too little fuel) if sufficiently large quantities of air are added. Dilution of the hydrogen-steam mixture with air does not eliminate the possibility of an explosion in a vacuum furnace. Likewise, the use of catalytic recombiners, as illustrated in U.S. Pat. Nos. 5,473,646 and 5,740,217 for nuclear power plants, are expensive, designed for use in hydrogen-lean hydrogen-air-steam mixtures, and may recombine the hydrogen too slowly to render the mixture nonflammable at all times.
The use of igniters to remove hydrogen through controlled combustion can overcome the previously mentioned disadvantages but the novel features of accident conditions in vacuum furnaces do not make this an obvious choice. The prior art describes the use of igniters only in nuclear power plants and not for the unique conditions encountered in the reactive metals processing industry. In nuclear power plants under accident conditions, the injected gas is a fuel, hydrogen, while in vacuum furnaces the gas drawn into the furnace is the oxidizer, air. No prior art describes the use of deliberate ignition of oxidizer jets. In nuclear power plants, the injected gas, hydrogen, is lighter than the surrounding air-steam atmosphere and floats. In the vacuum furnace, the air drawn into the furnace is heavier than the surrounding hydrogen-steam atmosphere and may sink or initially rise depending on the momentum of the incoming jet. Igniter placement is critical to controlled combustion and the prior art for the nuclear power industry does not provide any guidance for jets heavier than the surrounding atmosphere.
A device for controlled combustion of an ignitable hydrogen-air mixture in a nuclear power plant is described in U.S. Pat. No. 5,108,696. The device consists of an ignition source connected to a spark igniter. The ignition source has at least two different trip elements: one in response to a pressure rise and another in response to a temperature rise. These tripping elements are suitable for accident conditions in a nuclear power plant. However, for the unique conditions in a vacuum furnace, the mixture only becomes flammable when air enters the furnace.
Furthermore, the prior art does not describe a method by which the igniters should be used to mitigate a hydrogen explosion in a vacuum furnace. For example, igniters have been used in nuclear power plants but the geometry of a nuclear power plant is substantially different than that of a vacuum furnace. The nuclear power plant is surrounded by a very large containment divided into smaller compartments. A vacuum furnace, on the other hand, has a single melting chamber tank with a significantly smaller volume than that of a nuclear containment. Igniter placement is critical. A poorly placed igniter in a vacuum furnace would be no better than the random ignition sources that create the hydrogen explosions during accidents. The proper method when using igniters in a vacuum furnace can yield the difference between controlled combustion and a catastrophic explosion.
All prior art references for the controlled combustion of hydrogen during accident conditions are from a different technical field, the nuclear power industry. The devices, as described, are not suitable for the unique conditions associated with hydrogen explosions in the reactive metal processing industry and methods of their use to prevent explosions in vacuum furnaces are not described.